Various wireless mediums have no doubt made our lives easier and more convenient in a wide range of different ways over the years — but they certainly aren't perfect. Wireless transmissions, especially RF transmissions, are both noisy and unreliable by their very nature. In an effort to provide reliable data delivery, to control access to a naturally shared medium and to protect the integrity of the data being delivered in the first place, companies use techniques like the Clear Channel Assessment to help accomplish all of these things and more.
It sounds like a complicated topic, but in reality it's quite straightforward. You just need to keep a few key things in mind.
What is the Clear Channel Assessment? Breaking Things Down
To get a better idea of what a Clear Channel Assessment actually is, one must first get a better understanding of the CSMA/CA protocol — otherwise known as the Carrier Sense Multiple Access/Collision Avoidance protocol used on 802.11 networks.
The CSMA/CA is actually an alternative to the CSMA/CD protocol, which deals specifically with transmissions after a collision has already occurred. The CSMA/CA, as the name implies, attempts to stop these collisions before they actually happen.
When the CSMA/CA is employed, once a node receives a data packet for transmission, it first checks to make sure that the channel to be used is actually clear — meaning that no other node is transmitting at the time. Provided that the channel is free and clear, the packet will be sent. If another transmission is detected, the node will wait before checking again.
In other words, this is a standard used to define whether or not an RF (radio frequency) medium is busy in a carrier sense.
What all this has to do with the Clear Channel Assessment in particular has to do with Wi-Fi radios in an effort to make sure that broadcasts are not delayed or otherwise distorted when making their way to users. This type of physical carrier sense is constantly being performed by all Wi-Fi devices like radios, regardless of whether or not they happen to be transmitting at the time. All told, this actually has two distinct purposes:
- It is a concerted effort to determine whether a frame transmission is incoming that the station will then receive. If the particular medium in use happens to be busy, the radio will attempt to synchronize with that transmission.
- But more importantly, it's an attempt to find out whether or not the medium is busy BEFORE transmitting. Everything must be clear for the station to transmit in the first place.
In an effort to achieve both of these objectives at the same time, Wi-Fi radios using the 802.11 standard use what is called a Clear Channel Assessment, otherwise known as the CCA, to essentially "appraise" the RF medium that is about to be used.
The Clear Channel Assessment itself has a number of core components:
- The device will "listen" for RF transmissions at the physical layer. These types of radios actually use two separate CCA thresholds when doing this — the SD (signal detect) threshold is used to identify any transmissions from another 802.11 radio, while the ED (energy detect) threshold is about 20 dB higher than the SD threshold.
- The energy detect threshold in particular is used to detect any other type of RF transmissions that may be present during a Clear Channel Assessment that do not fall under the definition of those outlined above. This is important because both the 2.4 GHz and 5 GHz bands are "license-free," which means that other non-802.11 radio frequency transmissions could theoretically be occupying the channel at any time.
Depending on what information is uncovered during the assessment, the Wi-Fi radio in question will perform various functions in an effort to secure the best possible broadcast quality on behalf of users.
There are, however, a few things that can potentially complicate this — like the fact that the interpretation of these threshold values is largely up to the discretion of device manufacturers who produce WLAN and AP radios in the first place.
Likewise, the "receive sensitivity" of 802.11 radios can vary wildly from one model to the next. None of this means that end users necessarily have anything to worry about — it's just that the effectiveness of a Clear Channel Assessment can ultimately vary based on factors that are essentially out of their control.
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